Abstract: :
Purpose: The power and base direction of the prism requiredto relocate the retinal image on to functioning retina for subjectswith age-related macular degeneration (AMD) can be determinedsubjectively. This study investigated how well the requiredprism can be predicted from measures of visual function.Methods:We recruited 192 subjects (120 females and 72 males) with amean age of 80 years (range 56-97) to a randomised controlledtrial of prism relocation spectacles. Visual functions measuredincluded distance logMAR acuity (ETDRS charts), word readingacuity (Bailey-Lovie M scale), contrast sensitivity (Pelli-Robsonchart) and scotoma area (Bjerrum perimetry). Prism power andbase direction were determined using a subjective bracketingtechnique while the subject viewed a projected image subtending30°.Results: The average prism power was 7.3Δ (range 2to 15). Prism base direction in the superior hemi-field waschosen by most patients (63% versus 22% for inferior hemi-fieldand 15% exactly on the horizontal), with 90°commonly beingthe preferred position. The mean logMAR distance visual acuitywas 0.92 log MAR (range 0.42 to 1.60). Mean values for othervisual functions were 3.56 M for word reading (range 0.60 to10.00), 0.96 log CS for contrast sensitivity (range 0.05 to1.65) and 160 grid units2 for the area of the scotoma (range0 to 1529). Regression analyses indicate strong associationsbetween all visual functions and prism power; prism power wasestimated to increase by 0.37Δ per logMAR line (p<0.0001),by 0.24Δ per M scale unit (p<0.0001), by 1.8Δ per log CS unit(p<0.003) and by 0.32Δ per 100 grid units2 (p<0.0001).Conclusions: Subjectively prescribed prism power shows strongassociations with logMAR visual acuity, word reading acuity,contrast sensitivity and area of scotoma. However, the magnitudeof variance in prism power strength explained by visual functionvariables is relatively small, implying that for a large numberof patients, the predicted power would not closely match thepower obtained by the subjective method. The superior retinaappears to be preferred for prism relocation, suggesting improvedfunction for rehabilitation.